The invention relates to a motor-driven type power steering apparatus using a motor as a source of generation of steering-assisting power.
A motor-driven type power steering apparatus for a vehicle includes a motor for steering-assisting purposes, and a reduction gear mechanism for transmitting a rotational force of the motor to a steering unit. The power steering apparatus assists the operation of the steering unit corresponding to the rotation of the steering unit by the rotation of the motor so that the driver's labor, required for the steering operation, can be reduced.
The reduction gear mechanism includes a worm serving as a drive gear which rotates in accordance with the rotation of the motor, and a worm wheel serving as a driven gear which meshes with the worm.
In the motor-driven type power steering apparatus using such a reduction gear mechanism, in order to reduce the amount of backlash in meshed portions of the worm and worm wheel so as to reduce rattle noise due to the backlash during the steering operation, the worm, the worm wheel, bearings, a housing, etc., are suitably selected and assembled (so-called stratified assembly) so that the rotation-axis distance between the axis of rotation of the worm and the axis of rotation of the worm wheel can fall within an allowable range. However, much time is required for this assembling operation.
There is known the type of motor-driven type power steering apparatus (for example, JP-A-2002-96749) in which a shaft portion of a worm disposed close to a motor and a shaft portion of the worm disposed remote from the motor are rotatably supported on a housing through respective rolling bearings, and a rubber ring is mounted on an outer peripheral surface of the motor-remote-side shaft portion of the worm, and the worm is urged or pressed toward the worm wheel by an elastic restoring force of the rubber ring so as to reduce the rotation-axis distance, thereby reducing the amount of backlash in meshed portions of the worm and worm wheel.
However, in the related motor-driven type power steering apparatus of the above construction in which the amount of backlash is reduced by the use of the rubber ring, each time the steering operation is effected, a load and a rotational torque are applied to the rubber ring in a radial direction, and therefore the rubber ring is liable to be subjected to deterioration such as permanent set in fatigue. And besides, the degree of freedom for setting a preload by the urging force is relative low since the elastic restoring force of the rubber ring itself urges the worm.